1. Field of the Invention This invention relates to a radiation detection apparatus which contains at least one photomultiplier tube for the detection of light. In particular, this invention relates to a radiation detection apparatus such as a detector head for a gamma scintillation camera or a radionuclide emission computed tomography system.
2. Description of the Prior Art Photomultiplier tubes are widely used in radiation detecting apparatus such as a gamma radiation scintillation camera (see e.g. U.S. Pat. No. 3,011,057 Anger) and in both positron emission computed tomography or in positron ECT, (see e.g. IEEE Transactions on Nuclear Science, Vol.NS-23, February 1976, pp. 528-537) and single photon emission computed tomography, or SPECT (see e.g. IEEE Transactions on Nuclear Science, Vol. NS-28, February 1981, pp. 69-80). This invention applies to all such radiation detectors. For brevity, the term ECT subsequently is used for both SPECT and positron ECT systems.
It has been observed that the sensitivity of a photomultiplier tube varies with the position of the tube with respect to room coordinates. In particular, it has been realized that the sensitivity of a photomultiplier tube is a function of its orientation with regard to the magnetic field of the earth. Examinations also have proved that some types of photomultiplier tubes typically used for gamma camera applications are sensitive to magnetic fields of the order of 1Gauss (10.sup.-4 Tesla).
As far as photomultiplier tubes are concerned which are incorporated in a conventional scintillation camera, no specific measures have been taken to compensate for the influence of the earth's magnetic field. The reason herefor is that a change of the camera orientation does not have any immediate impact on the quality of the camera image. In a conventional stationary camera the magnetic field orientation does not change in any way while images are taken, and in a conventional scanning camera the orientation of the camera head with respect to the earth's magnetic field is not altered during a scanning operation of the head.
In the case of a single head SPECT system, a single camera head rotates about the patient to be examined. In a dual SPECT system or positron ECT system, two opposing camera heads rotate about the patient. In both types of ECT systems, an influence can be observed which is periodic with the rotation. It has been found that this influence is caused by the sensitivity of the incorporated photomultiplier tubes to the varying orientation with respect to the earth's magnetic field. The periodic perturbation of the tube sensitivity may cause artifacts in the reconstructed ECT images. These magnetic field induced artifacts may impede the analysis of patient images.
In new types of stationary cameras, and in ECT camera systems, manufacturers have designed sophisticated correction techniques correcting for linearity (that is, the LC part in a so-called ZLC camera), flood uniformity, and/or spatial energy variations (that is, the Z part in a ZLC camera).
Most of these techniques require the acquisition of a correction matrix, that is the acquisition of an XY-two dimensional representation of the amount of correction that has to be made. If the camera is operated in a position different from the position in which the correction matrix was established, image distortions or non-uniformity effects may occur, since the magnetic field in the operating position may be different.
Magnetic shielding may reduce magnetic field effects to a certain degree. However, shielding is usually not inexpensive because it involves shielding material which is costly and which requires some labor to be formed into special shapes. In general, very carefully designed shielding will be needed. It will be noted however, that it is difficult to shield adequately against axial fields, that is magnetic fields parallel to the photomultiplier axes. These axes are conventionally parallel to the axis defined by the collimator. In some cases, such on-axis shielding is not even possible, or it produces undesirable effects. In conventional gamma cameras, no shielding is used on the scintillator, with the exception of that provided by the collimator.
Another approach to overcome magnetic field problems would be to position the scintillation camera or the ECT camera in a shielded room or within the volume of large Helmholtz coils. This, of course, is also an expensive solution. Such Helmholtz coils could not compensate for field distortions produced by other smaller sources such as an electric motor, a solenoid actuator, or a transformer. As the field of view is increased in gamma cameras and ECT procedures become more refined, magnetic field effects will become more troublesome. Therefore, it seems advisable to find solutions which reduce these problems.